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\title{Antenna Calibration Techniques}
\author{Michael Moore}
\date{\today}
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PCVS were determined and analyzed in laboratory experiments from the very start of the development of GPS. 
Sims(1985) reporterd absolute values of elevation and azimuth dependent PCVS fo rthe antenna of the first civillian T14100 GPS receiver.

% Do nothing approach 1980 - 1990s? pure field technique
In practise the same types of antennas were used and antennas were orientated in the same direction, so that over short distances the PCV effect was not signficiant.
At this time is was sufficient to measure the height of the antenna to the same reference point.
It was not possible to distinguish PCV effects on static measurements over long periods at short to medium ranges from other effects.

\section{Relative Calibrations}
% Relative Calibrations offset only
When it became common practise to use different types of antennas and measurements were being made over long distances, the satellites had significantly different viewing anglesand the effects of different PCVS was becoming apparent.

Absolute calibration from anechoic chambers and their analysis are reported by Schpler (1994), Schupler et al. (1995) and Rohan et al (1996).
Obtaining the absolute PCV was a relatively costly lab calibration and did not give satisfactory results in practise.
(Scale bias issue?)

This led to the introduction of field procedures for antenna calibrations. 

They were able to estimate elevation dependent PCVs in relation to a reference antenna.
The PCVS of the reference antenna were assumed to be 0, and offsets were fixed to a predetermined value (Rothacher and Mader 1996).
The absolute PCV component of the reference antenna, with a variation of 28mm for the ionosphere-free linear combination (L0), is not corrected and introduces a systematic error (Wubbena et al 2000b).

Multipath is a critical factor in relative calibration procedures and there have been several attempts to minimise its effect by site selection and averaging over time.
The PCVS at this time were commonly applied from estimatations from an uncomplicated field procedure (Mader 1999).
Freqeuntly only manufacturers data on the mean L1 offset, sometimes the L2 value were used in the computations.
Alternatively offsets were determined in seperate rotation test measurements or taken from published calibration results. 
PCV corrections were generally not taken into account.

% elevation dependent PCVS
However due to the increasing use of different antenna types, and with the increase in number of permananet reference stations and RTK networks the need for full PCV corrections became apparent.

% field techniques
% Mader
% antenna swap technique, etc..
\section{Absolute Calibration}
% absolute calibration
Initially comparison of relative and absolute PCVS shows an effect emerges in global networks which represent a scale error of 15 ppb as a first approximation (Rothacher et al. 1995, Menge et al. 1998).
on the basis of results of measurement techniques other than GPS, absolute PCVs were first thought to be the source of this error. The cost of determining absolute PCVs in a lab inhibited further investigations.
Experiments have now shown that absolute PCVs can be excluded as the cause of the so-called scale errors in global networks. New investigations into other sources of error, such as the satellite antenna and troposhpere were required.
Some effort was made to show the difference in relative and absolute calibration techniques. (German benchmark test)

% anechoic chamber tests
\subsection{Anechoic Chamber}
The use of anechoic chambers can be fairly expensive to build and run.

% anechoic chamber Bonn
\subsubsection{Bonn anechoic chamber}
Calculated the minimum dimensions needed for a GPS antenna anechoic chamber

% Corrections made to the data, etc..

% Near-field issues
% Field technique - side real day

% Robot Technique
\subsection{Field Techniques}

\subsubsection{GEO++ and University of Hannover Field Technique}
Observation technique requires 6000 - 8000 different positions depending on time and satellite constellation.
It allows the recovery of azimuth dependent PCVs accurately and reliably.
One calibration takes approximately 3hours.
The measuring program is automated and the current satellite constellation is taken into account.
Need to calibrate the robot - used to have to use a theodolite, but now have a self calibration techniqe.

They use a dynamically changing elevation mask which will only observe sats above 18.

the observation program is always variable, so this reduces the probablilty of any systematic effects.

Techniques has been proven to work in all weather conditions, and at different stations. 

\subsubsection{NGS Field Technique}
% NGS automated technique
Time difference single difference technique


% future challenges
integrate a new robot with a higher payload this will allow for NF calibrations with monuments
develop a routine to be able to calibrate with only 1 or 2 sats for L5 or new signals, etc.
are there cheaper options that can be developed besides the GEO++?

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